Journal of Hydraulic Engineering

Two key factors in dam-break modeling are accuracy and speed. Therefore, high-performance calculations are of great importance to the simulation of dam-break events. In this study, we develop a two-dimensional hydrodynamic model based on the finite volume method to simulate the dam-break flow routing process. Roe’s approximate Riemann solution is adopted to solve the interface flux of grid cells and accurately simulate the discontinuous flow. A graphics processing unit (GPU)-based parallel method, OpenACC, is used to realize parallel computing. Because an explicit discrete technique is used to solve the governing equations, and there is no correlation between grid calculations in a single time step, the parallel dam-break model can be easily realized by adding OpenACC directives to the loop structure of the grid calculations. To analyze the performance of the model, we considered the Pangtoupao flood storage area in China using a Nvidia Tesla K20c card and four different grid division schemes. By carefully studying the implementation method and optimization of data transportation in the parallel algorithm, a speedup factor of 20.70 can be achieved. This acceleration is better than that of the OpenMP method with a 16-kernel computer. Further analysis reveals that models involving a larger number of calculations exhibit greater efficiency and a higher speedup rate. In addition, the OpenACC parallel mode has good portability, making it easy to realize parallel computation from the original serial model. This GPU-based parallel computation has the advantages of high performance and easily available required hardware.

The potential benefit of coupling hydraulic and electronic regulation to maximize the energy production of a cross-flow turbine in hydraulic plants is analyzed and computed with reference to a specific case. Design criteria of the cross-flow turbine inside hydraulic plants are first summarized, along with the use of hydraulic regulation in the case of constant water head and variable discharge. Optimal turbine impeller rotational speed is derived, and traditional as well as innovative systems for electrical regulation are presented. A case study is analyzed to evaluate the potential energy production according to the expected monthly mean flow distribution and two possible choices: CFT1 with the hydraulic regulation, and CFT2 with coupled hydraulic and electric regulations. The return time of capital investment (RCI), computed for both the solutions, showed that the CFT2 solution provides an increment of the total produced energy, along with an increment of approximately 30% of the corresponding RCI. The sensitivity of the results to water head variability and to possible different pipe design criteria in future scenarios is finally discussed.

Thanks to their efficiency and simplicity of contruction, breastshot water wheels represent an attractive low head hydropower converter. In this work, a breastshot wheel is investigated by numerical simulations, and the results are validated with experimental tests. The average discrepancy between the numerical shaft torque and the experimental torque is lower than 5%. The numerical model is then used to investigate the performance and the hydraulic behavior of the wheel for different numbers of blades (16, 32, 48, and 64 blades) and at different hydraulic conditions. The increase in efficiency from 16 blades to the optimal blades number ranges between 12 to 16% in function of the hydraulic conditions. Empirical laws are also reported to quantify the improvement in efficiency with the blades number. These laws can support the design process of similar breastshot water wheels. The optimal blades number for this kind of wheel is identified in 48.

Time-varying scouring upstream of a sediment release gate during a flood needs to be accurately predicted for reducing sedimentation around a water intake. This paper presents an ordinary differential equation–based model for computing time variation of scoured volume and maximum scour depth upstream of a slit weir for steady and unsteady flow. The model is based on the sediment continuity equation and sediment entrainment volume from a scour hole. There are some hypotheses in the model, and these hypotheses were examined by carrying out (1) 45 runs of scour inception tests, (2) 36 runs of numerical tests using computational fluid dynamics, and (3) 33 runs of physical scour tests measuring time variation of scoured topography. The model was validated with the scour tests for steady and unsteady flow conducted in the present study in addition to those reported in different research. The model was found to agree well with the data of time variation of the scoured volume and maximum scour depth, allowing for using a consistent numerical scheme irrespective of steady or unsteady flow condition.

This paper proposes a new approach based on the frequency response for fault identification in pipelines. The method focuses on extended blockage detection and location, and a case of simultaneous punctual blockage and leak identification. The proposed approach is based on a frequency modeling of pipeline dynamics including the effect of faults under study, by means of transfer matrix representation, and a related optimization: using measurements at both ends of the pipeline, together with an appropriate excitation, the parameters of the faults are directly identified from the frequency response. This methodology is shown to be very efficient based on the simulation results, even in the presence of additive measurement noise.

Recent experimental studies suggest that the usual way of measuring and predicting the efficiency of the cross-flow turbine do not describe the flow physics adequately. This verification led to the present investigation of some of the more relevant aspects related to the efficiency of this type of turbine. After presenting a detailed description of the rig used in the tests, the most adequate way of measuring the efficiency is discussed, including the specification of the proper datum used for the measurement of the head. Typical curves for the measured efficiency are shown, demonstrating that a peak efficiency of 84.8% is attainable with the cross-flow turbine, using new geometric relations, markedly different from those proposed by the present know-how in this field. Analysis of the experimental data seems to suggest that one of the reasons for the detected discrepancies is the assumption for the direction of the flow leaving the rotor. The consequences of the new physical insight are explored in this paper, leading to new theoretical relations for the efficiency evolution. It is demonstrated that the new theory describes reality better than previous analysis by comparing some of its predictions with experimental data.

Experimental investigations showing the mean and fluctuating velocity fields of a three-dimensional round wall jet in counterflow are presented. The ratios of jet to counterflow velocity ranged from 2.5 to 25 and the jet Reynolds numbers were from 1,000 to 10,000. The jet penetration length, which is the axial distance that the wall jet penetrates into the counterflow before stopping, was measured and scaled with a power-law relation. The decay of the jet velocity during its penetration into the oncoming flow was analyzed with proper scaling parameters. The results show that up to 70% of the wall jet penetration length, the velocity decay, and the spreading rate are effectively identical to those of a wall jet in quiescent ambient flow. Above this distance, the velocity decay is linear and the jet spreading rate increases sharply. The mean axial velocity profiles show self-similarity between 25 and 80% of the penetration distance and are similar to the case with no counterflow. The vortical structures from the jet and the opposing turbulent boundary layer interact in the stagnation area to provide a large recirculation region with stochastic oscillations that enhance the mixing of the wall jet and counterflow. These interactions result in a region of high turbulence intensity near the upstream extents of the wall jet. The profiles of Reynolds stresses are presented and discussed. It was observed that the normalized velocity fluctuation profiles show self-similarity for the region within 30 and 70% of the wall jet penetration length, which is shorter than the region for which the axial velocity profiles are self-similar.

This study numerically investigates the effect of various plunge pool lengths on the flow conditions with a fixed subcritical approach flow because the flow regimes of free overfall in a vertical drop pool can be mainly classified into skimming, nappe, and periodic oscillatory flows depending upon different discharges and pool dimensions. The simulated water surface and oscillatory period compare well with the experimental data. After the correctness of the numerical model is validated, the velocity and vorticity contours are examined to explore the surface jet of the skimming flow, the impinging jet of the nappe flow, and the dropping flow switching between these two jets. The effect of an air pocket underneath the dropping jet on the oscillatory behavior is addressed, as well as the kinetic and potential energy evolutions are discussed in view of control volume analysis during the periodic oscillatory process. Finally, the authors calculate the hydraulic head loss to evaluate the energy dissipation efficiency with various drop pool lengths for different flow regimes.

Although thousands of gabion check dams have been installed to control gully erosion in Ethiopia, several challenges still remain, such as the issue of gabion failure in ephemeral streams with coarse bed load, which abrades at the chute step. As an alternative for gabion check dams in torrents with coarse bed load, boulder-faced log dams were conceived, installed transversally across torrents, and tested ($n=30$). For this, logs (22–35 cm across) were embedded in the banks of torrents, 0.5–1 m above the bed, and their upstream sides were faced with boulders (0.3–0.7 m across). Similar to gabion check dams, boulder-faced log dams lead to temporary ponding, spreading of peak flow over the entire channel width, and sediment deposition. The results of testing under extreme flow conditions (including two storms with return periods of 5.6 and 7 years) show that 18 dams resisted strong floods. Beyond certain flood thresholds (represented by proxies such as Strahler’s stream order, catchment area, $D_{95}$, or channel width), 11 log dams were completely destroyed. Smallholder farmers see much potential in this type of structure to control first-order torrents with coarse bed load because the technique is cost-effective and can be easily installed.

To facilitate administrators’ abilities to handle reservoir operations, factors on how muddy water was formed inside the reservoir should be identified. According to field observation data, during typhoon events, there were big uncertainties caused by only using the reservoir inflowing discharges to evaluate turbidity condition of raw water inside reservoirs. The data also showed that time intervals between typhoon events could be relevant. The state of the bottom silt inside the reservoir and the erosion capacity of the inflowing turbidity current both might play key roles. In view of these facts, a flume experiment was conducted to analyze the threshold condition of bottom-silt entrainment by turbidity currents. Then, to evaluate and validate muddy water phenomena during storm rainfall events, an empirical equation for evaluating the concentration of reservoir bottom silt after a long time was established by a 4-year experiment on flocculent settling with silt-clay consolidation. The results showed that when the ratio of bed shear stress of turbidity currents to critical shear stress was large, peak turbidity formed by the inflowing turbidity current was relatively high; however, the correlation between that ratio and peak turbidity could be further strengthened by considering silt-clay consolidation time on the bottom concentration of reservoirs. This explained the different levels in terms of turbidity for rainfall events of similar scale.

A complete understanding of the role of grain-scale particle-flow interaction in sediment entrainment and transport has still not been achieved in spite of recent technological advancement in measurement capabilities. In this study, the initial motion of natural sediment particles in a gravel deposit was detected and combined with simultaneous local measurements of the velocities on a horizontal plane located above the bed surface using a three-component stereoscopic PIV. A series of experimental tests with increasing low values of boundary shear stress were conducted. The acquisition system allowed coupling between streamwise and vertical near-bed velocity and the entrainment of more than 900 individual grains. Initial analysis agreed with previous observations on the predominance of sweeps (Quadrant IV), and to a lesser extent, of outward interactions (Quadrant I) in entraining gravel particles. However, the latter were found to move sediments just as efficiently as sweeps impacting on particles that had long periods of rest and so were exhibiting higher levels of stability. This behavior suggests that sweep-induced lift based on Bernoulli’s principle does not entirely explain the generation of vertical forces on highly stable bed particles. Closer inspection of the data revealed that many entrainments were correlated to occasions when stable bed grains interacted with grains travelling in their close vicinity. Approximately 30% of the entrained population was observed to initiate motion in this type of situation. For this subsample of entrainment events, outward interactions were found to be comparatively more effective than for the noninterference case, whereas the relative contribution of sweeps exhibited an opposite trend.

This note examines a globally smooth approximation for friction loss in pipelines suggested by Burgschweiger et al. The approximation contains five parameters, two of which are open to selection. A method is provided for choosing these based on laminar conditions near zero flow. Using the proposed approach, approximation accuracy is reported in terms of relative roughness and Reynolds number.